1. Field of the Invention
The present invention relates to a process for the preparation of finely divided, porous and rapidly water swellable polysaccharide graft polymers. These polymers are prepared by semi-continuous reverse-phase suspension polymerization, partial dehydration and post-crosslinking.
2. Description of the Background
Water-absorbing polymers are in diverse use in the sanitary and hygiene sectors as water absorbents in paper napkins and paper towels, as tampons, patient undersheets and electrolyte thickeners in dry batteries, as moisture-retaining agents or water-storing agents in agriculture and as desiccants.
Suitable polymers are polysaccharide derivatives to which are usually grafted water-soluble vinyl monomers. Such polymers include carboxymethylcellulose, hydrolysed starch/acrylonitrile graft polymers, acrylic acid/starch graft polymers or fully synthetic, weakly crosslinked polymers such as partially crosslinked polyacrylic acid salts and partially crosslinked polymaleic acid derivatives.
A process for the preparation of resins having a high water absorption capacity is described in DE-C-26 12 846. In this process starch is also polymerized with acrylic acid and a crosslinking agent in aqueous or aqueous alcoholic solution. With solids contents of usually below 20%, however, rubber-like block gels are formed in this precipitation polymerization, from which the pulverulent end products are obtained only after drying and grinding.
According to Japanese Patent 80/139 408, a graft polymer can be prepared which can subsequently be hydrolysed and crosslinked by polymerization of acrylonitrile in an aqueous medium in the presence of starch. A powder having a water-absorption capacity of 150 to 180 ml/g is obtained.
A graft polymerization in suspension is taught in Japanese Patent 80/161 813. In this process a mixture of n-hexane with sorbitan monostearate, starch, water, acrylic acid, sodium hydroxide solution and water-soluble initiator is first prepared, before the polymerization is initiated by heating. However, in this case the reaction product tends to agglomerate during the polymerization and is not obtained in finely divided form.
According to DE-C-28 40 010 cationic, water-soluble polysaccharide graft polymers can be prepared by inverse suspension polymerization. In this process a polysaccharide is first suspended in the presence of a surface-active agent in a water-immiscible solvent. An aqueous monomer solution, which predominantly contains acrylamide and which can also contain acrylic acid in small amounts, is then added at room temperature. After addition of an initiator, the mixture is heated and polymerized. The solids content, based on the aqueous polymer mixture, is more than 50%. In this case no crosslinking agents are employed and the water-swellable gel-like polymers are not obtained. The batch-wise preparation process indicated leads, at the start of the polymerization, to temperature peaks which are difficult to control in the case of large batches.
In EP-B-O 083 022 acrylic acid is polymerized in the presence of starch in aqueous solution. The products can then be crosslinked in an inert solvent in the presence of 0.01 to 1.3 parts of water per part of resin. The preparation of the starch graft polymers is effected at a solids content of only 20%. Moreover, these starch graft polymers--crosslinked and non-crosslinked--have only a low water-absorption capacity.
DE-A-38 01 633 shows the preparation of polysaccharide graft polymers by inverse suspension polymerization, partial dehydration and crosslinking. The inverse suspension polymerization is, however, carried out in a single stage and discontinuously, by first adding together all reactants and then initiating the polymerization by heating in the presence of an initiator. With this process heat is instantaneously liberated at the start of the polymerization. In industrial production reactors a sufficiently rapid removal of the heat of polymerization can frequently be ensured only with difficulty. A need therefore continues to exist for a method of preparing polysaccharide graft polymers of good water absorptivity.